Breakthrough!
By renaming some files to match those listed in the "stdout" file (the one that appears when I run the .exe, not the one posted above), I managed to open a level in the editor by just double-clicking the .exe - no messing around with the command window. I've also figured out a way to open egyptian levels (and presumably shadow ones too).
EDIT: Changing the LEVELXXX.DAT file for a level to LEVEL000.DAT seems to be enough to make it work without using the command window - even for egyptian and shadow levels
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Hopefully now I can start making some levels of my own
Thanks Mindless for trying to help me out - much appreciated